This invention relates to a continuous process for producing heteropolysaccharides, and more particularly to a multi-stage continuous system for producing heteropolysaccharides.
In recent years considerable interest has been exhibited in heteropolysaccharides produced by the bacterial fermentation of carbohydrates. This interest has been greatly increased by the discovery that certain heteropolysaccharides formed by biochemical synthesis have properties which permit their use as thickening agents for water used in secondary recovery operations carried out in the petroleum industry, and also as food-grade ingredient for thickening such products as salad dressing. It has been found that some of these materials added to water or brine in suitable concentrations produce viscous solutions which are relatively stable under the conditions which prevail in subsurface oil reservoirs and food substances. By utilizing a solution of controlled viscosity in place or in addition to the water or brine normally employed in waterflooding projects, a favorable mobility ratio between the oil in the reservoir and the liquid used to displace it can be obtained. The tendency of the displacing medium to channel through highly permeable sections of the reservoir without displacing oil from the less permeable sections is greatly decreased. Viscous forces which normally reduce the displacement efficiency in portions of the reservoir through which the displacing medium actually passes are more readily overcome. As a result of these effects, the use of water or brine containing polysaccharide thickening agents generally permits the recovery of significantly greater quantities of oil during waterflooding than can be removed with water or brine alone.
A particularly effective polysaccharide for use as a thickening agent during oil field waterflooding operations is the heteropolysaccharide produced by the growth and enzymatic action of bacteria of the genus Xanthomonas upon sugar, starches and similar carbohydrates. This material is more commonly know as "Xanthan gum". Studies and comparitive tests have shown that this material, a polymer containing mannose, glucose, glucuronic acid salts, and acetyl radicals has much greater thickening power than dextran and similar polysaccharides and hence can be used in significantly lower concentrations than the other materials. It is effective in both fresh water and brine and has excellent high temperature stability characteristics.
Although the heteropolysaccharide, i.e., "Xanthan gum", has superior qualities and characteristics for use in the petroleum industry, the total capacity for its production is far below its requirement. In fact, the present capability for its production is less than 20% of its estimated requirement in the near future.
Thus, there is a need for processes and systems which will greatly increase the production of the heteropolysaccharides to satisfy the requirements in the petroleum industry. In addition, there are other industries which require significant amounts of "Xanthan gum" such as the food industry where it is used as a stabilizing agent and/or thickener.
The present invention, as described below, is intended to satisfy this need by providing a continuous system for the production of heteropolysaccharides, i.e., Xanthan gum, which is efficient, effective, and relatively cell-free.